1. Technical Field
The invention relates to methods and systems for broadcasting high definition television programs, called HDTV, for providing an image having a much higher number of lines and pixels per line than in present standards. The European project EUREKA EU95 specifies for example an image at the frequency of 50 Hz, having 1250 lines instead of 625 lines and twice the number of pixels per line.
To avoid any ambiguity, it should be noted that an "image" is formed by several "frames" whereas in Great-Britain the notations "frame" for "image" and "field" for "frame" are often used.
It is important for the HDTV programs, at least during a transitory period, to be able to be transmitted over available broadcasting channels and received by existing receivers. In particular, the problem will arise of broadcasting HDTV programs over a MAC-packet television channel from a satellite. The MAC standard in fact fixes the channel width at 20.5 MHz, i.e. a value much less than that required for HDTV image transmission.
2. Prior Art
Different data compression processes have been proposed for reducing the bandwidth required for the HDTV image to the value which is available. These processes involve sub-sampling at transmission and interpolation at reception. Thus, a high definition image with low time activity (i.e. where the modifications are small from one image to the next) may be conveyed in a narrow transmission channel, by spreading the spatial data of an image over several successive television frames. Purely spatial shuffling techniques make it possible to broadcast all lines of a high definition image by scanning with a smaller number of lines. The left hand part of FIG. 2 shows a vertical shuffling technique applied to the standard structure of lines of the four frames shown in FIG. 1, samples x, o, y and a corresponding respectively to four successive frames numbered 1, 2, 3 and 4. Such a process for narrowing the required band is disclosed in European 0082,489. A wide band HDTV signal with sequential scanning is converted into an interlace scanning signal for transmission through a channel having a width which is too narrow for direct transmission of the sequential signal. Scanning conversion makes use of picture subsampling which may detrimentally affect the image. Spatio-temporal filtering is used for limiting the passband of the source signal and inhibiting the conversion defects. Low pass temporal filtering for limiting the amount of flicker completely removes high frequency information and makes it impossible to reconstruct it, even approximately, on reception.
But the sub-sampling and the shuffling are encoding operations which affect the quality of the "compatible" image, this term designating any image delivered by a first generation receiver (not deigned for receiving HDTV) fed via a transmission or broadcast channel with programs encoded by an HD encoder. The defects are due mainly to the aliasing due to subNyquist sub-sampling and to shuffling of the samples. Reference may be made in this connection to the article "Analyse de structures de sous-e spatio-temporel d'un signal TVHD en vue de sa transmission dans un canal MAC", HDTV Conference, Ottawa, October 1987.
The transmission of a stationary HD image over a reduced width channel results therefore, in particular, in time activity of the compatible image, which degrades it.
It might be thought that the compatible image could be improved by conventional spatial filtering methods. In fact, it appears that techniques for suppressing certain spatial frequencies cause a loss of spatial resolution in the compatible image, do not correct all the defects and have a low resistance to transmission noises. There continue to exist in particular certain effects of the sub-Nyquist sub-sampling, particularly aliasing and line scrambling effects which result in movements of the contours.